In modern aircraft construction it is usual, for visually aesthetic reasons amongst others, as well as for the prevention of corrosion effects, to coat the primary structure of aircraft with at least one polymer layer, for example in the form of paintwork. The polymer layer or layers can fulfil a large number of different functions. For example, a polymer layer can be a decorative coat, a top coat, an adhesion promoter, a filler coat, an anti-static coat, or an anti-erosion coat.
In the case of large passenger aircraft, whose primary structure can feature a surface area of several 1,000 m2, the mass of an applied polymer coating can be of the order of up to several 100 kg. So as not to increase the weight of the primary structure unnecessarily, the material thickness of an applied coating must on the one hand be as even as possible over the whole surface area of the primary structure. Apart from the requirement of an application with an even coating thickness, the material thickness of the coating must not be too high, so as not to impair the hardening process and to avoid the formation of wrinkles, bubbles and/or droplets, and the occurrence of other coating defects. On the other hand the coating thickness of the paintwork must also not turn out to be too small, if the desired functional effects are to be achieved. In general therefore for a complete coating of the primary structure of an aircraft, which as a rule is formed from a multiplicity of polymer layers or coatings, each fulfilling different functions, a material thickness of between 0.05 mm and 0.5 mm is selected. In order to be able to monitor this requirement with its close tolerances, the introduction of coating thickness measurement equipment is essential.
Known from the prior art are firstly purely mechanically acting coating thickness measurement units, with which the determination of the material thickness of the applied polymer coating takes place, for example, by means of the measurement of the penetration path of a measurement body. Such coating thickness measurement units do not allow on the one hand a non-destructive measurement and on the other hand the measurement accuracy that can be achieved is inherently limited in principle, in particular in the case of low material thicknesses of the polymer coating.
Furthermore coating thickness measurement units are of known art, in which the measurement of the material thickness of a polymer coating takes place with eddy current measurements with the introduction of electric coils. While measurement units of this type do allow a non-destructive measurement with a sufficiently high accuracy after calibration has taken place, they have the disadvantage that the substrate with the polymer coating must feature a really high electrical conductivity. Accordingly these units are only suitable to a limited extent for the measurement of the material thickness of a polymer coating on a substrate formed from a composite material, which as a rule has a relatively poor electrical conductivity, as, for example, in the case of a carbon fibre-reinforced epoxy resin.
From the German utility patent DE 1 792 402 a device for the non-destructive measurement of a coating thickness of a medium on a base material is of known art, in which the measurement is based on the evaluation of the phase displacement that ensues with the passage of microwaves through the medium and their subsequent reflection from the base material. However this coating thickness measurement unit of prior art only allows a measurement of the thickness of a coating located on a purely metallic base material. Moreover the dielectric constant ∈ of the coating to be measured enters into the result of the measurement with the factor √{square root over (∈)}.